logcat

The Android log reading tool is named logcat. This application is available on all Android devices.

A good overview of using logcat is at http://developer.android.com/guide/developing/tools/adb.html#logcat. Please get familiar with this useful tool first.

logcat provides logs:

• from all the running Android applications (thanks to 4 logcat user space buffers)
• system
• main
• radio (for modem interface logs)
• events

 

Android loglevel

• There are different log level in Android : VERBOSE, DEBUG, INFO, WARN, ERROR, ASSERT, or SUPPRESS

• A different TAG per application is used, and the log level can be changed for each TAG with different method depending on how it is implemeted in the code.

• The default log level is INFO.

 

Dynamic log level implementation

• If implemented dynamically in Java with isLoggable, you can use :

adb shell setprop log.tag.<YOUR_LOG_TAG> <LEVEL>

or you can also create a local.prop file with the following in it: 'log.tag.<YOUR_LOG_TAG>=<LEVEL>' and place that in /data/local.prop, and reboot to apply.

 adb shell
 # echo "log.tag.<YOUR_LOG_TAG>=<LEVEL>" >> /data/local.prop
 # reboot

 

Static log level implementation

• If implemented statically in C/C++ or Java :

You need to check the code to know how the logging is implemented in the application.

Here is an example to enable verbose logs (logv) in a C/C++ application

• Those files already has these lines included to support LOGV:

#include <log.h>
#define LOG_TAG "tag_name"
//#define LOG_NDEBUG 0 

LOGV() function call at multiple places

• To see LOGV ouputs, you simply need to uncomment the #define LOG_NDEBUG 0 line. After that, rebuilt your image, boot to image again, and logcat would output logv messages.

 

Redirect stdout to logcat

• For Java/Android applications using System.out calls :

% adb shell setprop log.redirect-stdio true

To just view stdout output in logcat:
% adb logcat System.out:I *:S

• For native applications, you should use logwrapper

root@android:/ # logwrapper
Usage: logwrapper [-d] BINARY [ARGS ...]

Forks and executes BINARY ARGS, redirecting stdout and stderr to
the Android logging system. Tag is set to BINARY, priority is
always LOG_INFO.

-d: Causes logwrapper to SIGSEGV when BINARY terminates
    fault address is set to the status of wait()

     Simple use to redirect getevent output to logs :

  # logwrapper getevent

 

Print your message in logs

This could be really usefull to print your own message in logs from a shell to get temporal markers of what you have done.

root@android:/ # log
USAGE: log [-p priorityChar] [-t tag] message
        priorityChar should be one of: v,d,i,w,e

Simple example :

# log -p i -t MY_TAG "doing a test"

And you will get this in logs :

02-01 13:35:06.890 12746 12746 I MY_TAG  : doing a test